Sedimentary deposition of photoresist on semiconductor wafers

ABSTRACT

A conformal, substantially uniform thickness layer of photoresist is deposited on a semiconductor wafer by causing photoresist solids to &#34;sediment&#34; out of solution or suspension. Generally, the more conformal the layer, the more uniform the reflectance of the layer and the less variation in underlying feature critical dimension (cd). In order to accommodate possible resulting deviations in photoresist layer thickness causing undesirable reflectance nonuniformities (and cd variations), a top antireflective coating may be applied to the photoresist layer. In the case of a point-by-point lithography process, such as e-beam lithography, the thickness/reflectance variations can be mapped, and exposure doses adjusted accordingly.

This application is divisional of application Ser. No. 07/906,902, filedJun. 29, 1992 now U.S. Pat. No. 5,320,864.

TECHNICAL FIELD OF THE INVENTION

The invention relates to the fabrication of integrated circuit (IC)semiconductor devices, and more particularly to depositing a thin layer(film) of photoresist over a semiconductor wafer (substrate), moreparticularly a "topographical" (irregular front surface ) wafer.

BACKGROUND OF THE INVENTION

Photolithography is a common technique employed in the manufacture ofsemiconductor devices. Typically, a semiconductor wafer is coated with alayer (film) of photo-sensitive material, such as photoresist. Using apatterned mask or reticle, the wafer is exposed to projected light,typically actinic light, which manifests a photochemical effect on thephotoresist, which is subsequently chemically etched, leaving a patternof photoresist "lines" on the wafer corresponding to the pattern oflines on the mask.

This is all good in theory, until one recognizes that the uniformity ofthe illuminating light varies, typically at the source of the light, andthat such non-uniformity will manifest itself as variations in the sizeof features (e.g., photoresist lines) that can be created on the wafer.To the end of uniformizing the light incident on and passing through themask, various techniques have been proposed, among these a techniquediscussed in commonly-owned U.S. Pat. No. 5,055,871 (Pasch).

The ultimate goal of uniformizing (homogenizing) the incident light isthat the illumination uniformity (i.e., non-uniformity) ofphotolithographic apparatus will often set a limit to how small afeature, such as a line, can be imaged in a manufacturing environment.And, as a general principle, being able to create smaller integratedcircuit features is better (faster, more compact, etc.).

Of no less concern than the ultimate size (smallness) of features, isthe ability to control the critical dimension ("cd") from one feature toanother. For example, since size generally equates with speed (e.g.,smaller is generally faster), it is disadvantageous to have one feature,such as a polysilicon ("poly") gate, smaller (and faster) than anotherpoly gate on the same device. Conversely, it is highly desirable tofabricate all similar features (e.g., poly gates) to be the same size(i.e., with the same "cd"), especially in gate-array or cell typedevices, and combinations thereof, such as Application SpecificIntegrated Circuits (ASICs), including ASICs with on-chip memory.

In addition to nonuniform illumination, another cause of variation in"cd" is nonuniformity of the thickness of films overlying an irregulartopography on the wafer surface. Prior to the numerous steps involved infabricating integrated circuit devices on a semiconductor wafer, thewafer is initially fairly flat--exhibiting a relatively regulartopography. However, prior structure formation often leaves the topsurface topography of the silicon wafer highly irregular, with bumps,areas of unequal elevation, troughs, trenches and/or other surfaceirregularities. As a result of these irregularities, deposition ofsubsequent layers of materials could easily result in incompletecoverage, breaks in the deposited material, voids, etc., if it weredeposited directly over the aforementioned highly irregular surfaces. Ifthe irregularities are not alleviated at each major processing step, thetop surface topography of the surface irregularities will tend to becomeeven more irregular, causing further problems as layers stack up infurther processing of the semiconductor structure.

As mentioned above, the application and patterning of photoresist istypically a key step in the fabrication of complex integrated circuitdevices, and is a procedure that may be repeated at several differenttimes throughout the fabrication process.

It has been noticed, and is generally known, that the thickness of asubsequently applied film, particularly a layer of photoresist, willvary (in a generally non-predictable manner) depending upon theirregular topography of the underlying surface. (Application of anoverlying film to a flat, regular surface is generally not a problem.)For example, a photoresist layer, even if spun-on, will exhibit adifferent thickness, from point-to-point over the wafer (and within thearea of a given device) depending on the irregular topography ofunderlying features.

This variation in the thickness of photoresist over an irregulartopography is graphically illustrated in FIG. 1A.

FIG. 1A shows a portion of a semiconductor wafer 110 which has beenprocessed to develop raised structures 112a, 112b and 112c of FieldOxide (FOX), between which are active areas (islands) 114a and 114bhaving a lower elevation (e.g., at wafer level). The island 114a betweenthe FOX structures 112a and 112b has a width w₁ substantially smallerthan the width w₂ of the island 114b between the FOX structures 112b and112c. Having islands of different widths is not uncommon. For example,the island 114a may be an "array island" having a width w₁ on the orderof 3 microns, and the island 114b may be an "I/O (Input/Output) island"having a width w₂ on the order of hundreds of microns. Both types ofislands are usually required for an integrated circuit device, and it isnot uncommon to have widely varying island sizes in a single device. Inany case, the island areas are usually lower (less elevated) than thefield oxide areas.

An overlying layer 120 of polysilicon ("poly") is deposited over thewafer, which already exhibits an irregular topography. This is accordingto known techniques, and is presented herein by way of example only.

An overlying film (layer) 130 of photoresist is applied, in any suitablemanner, over the poly layer 120, and photolithographically treated tocreate etch-resistant "lines" (photochemically-converted areas) 132 and134 over the active areas 114a and 114b, respectively. The line features132 and 134 are shown in reverse cross-hatch from the remainder of thefilm 130.

Ultimately, the photoresist layer 130 is etched (or washed) away,leaving only a pattern of photochemically-converted areas 132 and 134overlying the poly 120. In subsequent fabrication steps, the wafer isetched (chemical, plasma, etc.), so that all but discrete poly regions122 and 124 (shown in reverse cross-hatch) underlying respectivephotoresist features 132 and 134, respectively, are removed from thesurface of the wafer. With additional processing, not shown, the polyregions 122 and 124 may perform as gates.

Since electron flow in the lateral direction (i.e., plane of the wafer)is of primary concern in the performance of circuit elements (e.g., polygates), the transverse dimension of the poly gates 122 and 124 parallelto the plane of the wafer is of paramount interest. For purposes of thisdiscussion, this transverse dimension is termed a "critical dimension"or "cd" The poly gate 122 has a first cd, designated "cd1", and the polygate 124 has a second cd, different from the first cd, designated "cd2".

In essence, the cd's of the two poly gates are different, because thewidth of the respective overlying photoresist features is different.(Generally, the width of a poly gate will be essentially the same asthat of the overlying resist feature.)

As mentioned above, it is nearly impossible to apply a uniform layer ofphotoresist over an irregular surface. Hence, the thickness of thephotoresist 130 over the active area 114a (particularly over the areawhere the poly gate 122 is to be formed) is different (shown thicker)than the thickness of the photoresist 130 over the active area 114b(particularly where the poly gate 124 is to be formed).

It is also generally known, that the reflectance of a film (such asphotoresist) will vary with its thickness. Hence, since the thickness ofan overlying film at any given point on the surface of the semiconductorwafer (e.g., photoresist) is not uniform, the reflectance isconsequently nonuniform from point-to-point across the surface of thewafer.

This indeterminate nature of resist thickness and reflectivity overirregular underlying surfaces has important, negative ramifications inthe semiconductor fabrication process, especially in the process offabricating circuit elements having "critical dimensions".

FIG. 1B illustrates the reflectivity problem, and its manifestation inthe size of a photoresist feature. Here, in a photolithographic process,a film 140 of photoresist is exposed to light (arrows ↓↓↓↓) ofhypothetically uniform intensity. A mask 150 is interposed in the lightpath, and is provided with light-transmitting areas (lines) 152 and 154allowing light (↓↓) to impinge upon selected areas 142 and 144,respectively, of the film 140.

In FIG. 1B, the thickness of the film 140 is intentionally shown to bedifferent in the areas 142 and 144. And, as will be seen, it isrelatively insignificant that the film is thicker in the area 144 thanin the area 142. For purposes of this discussion, depth of field (depthof focus) issues that may arise from projecting a mask image onto asurface of varying height may be ignored.

FIG. 1C illustrates graphically the effect of film thickness (horizontalaxis) on reflectivity (i.e., the energy reflected by the film), andrelates to the issues raised in FIG. 1B. While there is a generalincrease in reflectivity with increased thickness, there is a much moreprofound (generally sinusoidal) pattern of "maxima" 170a, 170b and 170cand "minima" 172a, 172b and 172c, which exhibits that the reflectivityfor a given greater film thickness (point 172c) can well be less thanthe reflectivity for a given lesser film thickness (point 170b). (Dashedhorizontal line 174 provided as a visual aid.) Importantly, thesevariations are dependent on relatively small, e.g., a quarter of awavelength, variations in the thickness of the film--difficultdimensions to measure, let alone control.

Returning to FIG. 1B, it can be appreciated that it is ratherindeterminate how much of the (supposedly uniform) incident energy (↓↓↓)will be absorbed by the photoresist film, and how much will bereflected, at any given point. And, as a general proposition, the moreincident energy (↓↓↓) that is absorbed at a given point, the greater thearea of the given feature 142 or 144 will "grow". Of course, the reversewould be true for reverse masking, wherein light acts outside of thedesired feature, in which case the more light absorbed--the smaller thefeature would be.

In any case, the point is made that an irregular thickness of anoverlying film (e.g., photoresist) will impact upon the ultimatecritical dimension (cd) of underlying features (e.g., poly gate) beingformed, with commensurate undesirable functional effects.

Certainly, if reflectivity issues were ignored, which they cannot be,the widths of all of the photoresist lines and underlying features wouldbe well-controlled. However, because the photoresist thickness variesfrom point-to-point over the wafer, and consequently its reflectivityvaries from point-to-point, the efficiency of the incident light on thephotoresist layer will vary commensurately, which will affect theultimate width of the resist features.

Evidently, the efficiency of the photolithography process is dependenton the ability of the photoresist material to absorb the radiant energy(light), and this ability is, in turn, affected by thethickness/reflectance of the photoresist.

In the prior art, it has been known to compensate approximately forknown variations (and to some extent, gross trends can be predicted) inphotoresist thicknesses by "differentially biasing" the (photomask) linewidths in the high versus low reflectivity areas. And FIG. 1Cillustrates that, to some extent, one can reasonably assume that theaverage reflectivity for an area with greater film thickness willreflect more than an area of lesser thickness. This concept may beemployed with respect to relatively large Input/Output (I/O) areas(e.g., 114b) versus relatively small active areas (e.g., 114a).

And, as mentioned before, in the prior art, it has also been known touse "spin-on" or other techniques in an attempt to apply a film (e.g.,photoresist) having a relatively planar top surface. Of course, arelatively planar top surface is of little help, and may in fact beantithetical, in uniformizing the thickness of a film over an underlyingsurface having an irregular topography--in which case the thickness ofthe film would vary widely from point-to-point. What is really needed isa "conformal" layer, in other words one exhibiting uniform thickness(not necessarily planar) over an irregular wafer surface.

It has also been known to reduce the viscosity of the photoresist sothat it goes on in a more planar manner. But, thickness will varyaccording to the topography of the underlying surface. Further, changingthe photoresist chemistry (viscosity) can have adverse side effects,such as poor photolithography resolution.

In the prior art, it has also been known to perform subsequent steps toplanarize the photoresist, which can be somewhat effective in overcomingthe reflectance issues set forth above--again, so long as thephotoresist is planarized over a relatively planar underlying surface.

Prior art techniques for accommodating "cd" variations due tophotoresist thickness variations are relatively difficult and timeconsuming to implement, and may not deliver the desired results.

The following U.S. Pat. Nos., incorporated by reference herein, arecited of general interest: 4,977,330; 4,929,992; 4,912,022; 4,906,852;4,762,396; 4,698,128; 4,672,023; 4,665,007; 4,541,169; 4,506,434; and4,402,128.

DISCLOSURE OF THE INVENTION

It is a general object of the present invention to provide improvedphotolithographic (or microlithographic) techniques for the fabricationof semiconductor devices.

It is another object of the present invention to provide a technique forobtaining a uniform film thickness, hence reflectivity of photoresistand/or other masking chemicals, regardless of the topography of anunderlying surface, particularly variations in the underlying activearea size.

It is another object of the present invention to provide a technique forimproving linewidth and cd (critical dimension) uniformity inphotolithography (microlithography), without using spin-on techniquesand without altering photoresist chemistry.

According to the invention, a "conformal" layer of photoresist isapplied (deposited) as a sedimentary layer over an irregular wafer. Thesedimentary layer of photoresist is conformal in that it is ofsubstantially uniform thickness irrespective of the topographicalvariations in the underlying wafer surface. Being conformal, thephotoresist layer will exhibit more uniform reflectivity, hence thecritical dimension (cd) of underlying features (e.g., polysilicon gates)can better be controlled.

In one embodiment of the invention, a semiconductor wafer is immersed ina solution of photoresist (solids) and solvent. The photoresist solidsare then allowed or caused to sediment (precipitate) out of solution,onto the surface of the wafer. A supersaturated solution of photoresistsolids in solvent may be employed. An "anti-solvent" may be added to thesolution to cause sedimentation. (An anti-solvent is a compound that"reverse biases" the polarity of the solvent, causing the dissolvedsolids to precipitate out of solution.)

In another embodiment of the invention, a semiconductor wafer isimmersed in a suspension of photoresist (solids) and water, preferablyde-ionized water. The photoresist solids are then allowed or caused tosediment (precipitate) out of suspension, onto the surface of the wafer.According to a feature of this embodiment, a film of water overlying thephotoresist (on the wafer) acts as a top antireflective coating.

By applying a substantially conformal (more uniform thickness) layer ofphotoresist over a wafer, the reflectivity of the layer is substantiallyuniformized. Hence, better control over underlying gate size can bedirectly effected, whereas differential biasing is considered to be asomewhat indirect approach to the problem.

Further according to the invention, the wafer is immersed so that itsfront surface (upon which devices are fabricated) faces up, whenimmersed. The wafer may be immersed either: (1) at the bottom of areservoir containing either a solution or a suspension of photoresistsolids; or (2) at a predetermined level in the solution/suspension.Hereinafter, when referring to either the solution or the suspension ofphotoresist solids, the term "mixture" will be employed, as embracingboth possibilities. Further, the term "liquid" will be employed, asembracing both solvents (for solutions) and water (for suspensions).

The wafer may also be "immersed" without a reservoir by coating thefront face of the wafer with a solution or suspension (mixture) ofphotoresist solids, then causing the solids to sediment (precipitate)onto the wafer, then washing (removing) the excess mixture off of theface of the wafer. For example, a mixture having photoresist solidscould be spun on to the face of the wafer.

In the case that the wafer is immersed at the bottom of a reservoircontaining a mixture of photoresist solids, the ultimate thickness ofthe photoresist layer on the wafer is preferably determined bycontrolling the "residence time" of the wafer in the mixture.

In the case that the wafer is immersed at a predetermined level in themixture of photoresist solids, the ultimate thickness of the photoresistlayer on the wafer is preferably determined by the amount of mixtureabove the wafer, assuming that all of the photoresist solids in thisportion of the mixture above the wafer will precipitate onto the surfaceof the wafer.

By applying the resist as a layer having substantially uniformthickness, reflectivity is uniformized, absorption of incidentphotolithographic light is uniformized, and better control over featurecritical dimension (cd) is effected, especially the cd of features indifferent topographical areas (e.g., active areas versus I/O areas), orbetween different sized transistors on an integrated circuit.

In another embodiment of the invention, a thickened mixture (suspensionor solution) of photoresist is applied directly to the wafer, withoutusing a reservoir, and the photoresist solids are caused to sediment outof the mixture as a conformal layer of photoresist onto the wafer.

Other objects, features and advantages of the invention will becomeapparent in light of the following description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a generalized, cross-sectional view of a portion of asemiconductor device (or wafer), and illustrates an exemplary problem inthe prior art, which problem is specifically addressed by the presentinvention.

FIG. 1B is a stylized cross-sectional view of a film (layer) of varyingthickness, as first discussed with respect to FIG. 1A.

FIG. 1C is a generalized graph of reflectivity (reflected energy) versusfilm thickness, and relates to FIG. 1B.

FIG. 2a is a cross-sectional, partially-perspective view of anembodiment of the invention.

FIG. 2b is a cross-sectional view of a wafer having photoresistdeposited sedimentarily, and an overlying top antireflective (TAR)coating, according to the present invention.

FIG. 3 is a cross-sectional, partially-perspective view of anotherembodiment of the invention.

FIG. 4 is a cross-sectional view of yet another embodiment of theinvention.

FIG. 5 is a perspective view of yet another embodiment of the invention.

FIG. 6 is a cross-sectional view of another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A-1C have been discussed above, and illustrate the impact thatnonuniform film thickness (e.g., nonconformal layer of photoresist), andconsequent varying film reflectivity, can have, especially in thephotolithographic fabrication of polysilicon gates using a patternedphotoresist layer (film).

According to the invention, the problem of nonuniform film (e.g.,photoresist) thickness exhibiting varying reflectivity is solved,generally, by sedimentary deposition ("precipitating") of photoresist onthe front surface of a wafer. Sedimentary deposition is accomplished inone of two manners:

1. Creating a solution, preferably supersaturated, of photoresist solidsand solvent, and causing the photoresist to sediment onto the surface ofthe wafer; or

2. Creating a suspension of photoresist and water, preferably deionizedwater, and allowing the photoresist to sediment onto the surface of thewafer.

DEPOSITION OF PHOTORESIST

Evidently, if a layer of photoresist, or other suitable photoreactivematerial, could be applied conformally, with uniform thickness,irrespective of the underlying topography of the substrate, andespecially in instances where the underlying topography is irregular,the resulting thickness of the photoresist film would be relativelyuniform and would exhibit relatively uniform reflectivity. Hence, thecritical dimension (cd) of underlying features being created with thephotoresist would tend to be more uniform.

As mentioned hereinbefore, one of the "best" known techniques forapplying a film of photoresist is the "spin-on" technique, which strivesto create a relatively flat (planar) top surface for the photoresistfilm. However, having a flat top surface is certainly no guarantee ofhaving a uniform thickness over an irregular underlying topography. Tothe contrary, having a planar top surface photoresist film over anunderlying irregular topography, however this may be achieved is, inmany cases, counter-indicative of having a uniform film thickness.

Other, non-related semiconductor fabrication processes teach thatvarious semiconductor materials can be deposited conformally. Forexample, chemical vapor deposition (CVD) of silicon nitride ("nitride")creates a "blanket" nitride layer which can cover an underlyingirregular (non-planar) surface with surprising uniformity of thickness.

By way of further example, U.S. Pat. No. 5,075,257 (Hawk, et al.),incorporated by reference herein, discloses that silicon may beaerosolized and electrostatically deposited onto various grounded, highmelting point substrates. Various preferred parameters are disclosed,relevant to aerosolizing silicon, including silicon powder size andpurity, velocity, spacing (from substrate), electrostatic charge level,temperature and time (heat cycle). The references cited in the Hawk etal. patent, are primarily directed to: other techniques of applyingsilicon, especially molten silicon;vaporizing/condensing/re-vaporizing/recondensing solids, especiallysilicon; and electrodeposition. Each material and each process havetheir own vagaries and solutions.

TOP ANTIREFLECTIVE COATINGS

Evidently, if a layer of photoresist, or other suitable photoreactivematerial, having an irregular thickness, could be caused to exhibituniform reflectivity, the resulting line widths formed therein byphotolithography would be more uniform. Hence, the critical dimension(cd) of underlying features being created with the photoresist would bemore uniform.

It is therefore known to apply a top antireflective coating (TAR) to theirregular top surface of photoresist over an irregular underlying layer.

Antireflective films, generally, have been known since Fabry/Perot,i.e., about one hundred years, for coating lenses (optics) and the like.Generally, an antireflective film has a refractive index less than thatof the material it is coating, and has a thickness of L/4 (one quarterthe wavelength "L" of the incident light in the underlying material).The wavelength "of choice" in photolithography is 365 nanometers (nm).

Preferably, according to Fabry/Perot, the refractive index "n_(TAR) " ofthe antireflective coating would be the square root of the refractiveindex "n_(PR").

Common photoresist solutions (i.e., 90% ethyl lactate solvent and 10%novolac resin solids) have a refractive index n_(PR) of approximately1.70, the square root of which is approximately 1.30. Hence, the idealTAR would have a refractive index n_(TAR) of 1.30.

Water, having a refractive index of approximately 1.30, would thereforemake an ideal TAR. Sugar also has a desirable refractive index, butwould be expected to crystallize (not desired) on top of photoresist. Aswill be discussed in greater detail hereinbelow, water may be employedas a top antireflective coating, and is suitably obtained as a byproductof precipitating photoresist solids out of suspension.

International Business Machines (IBM) is known to use a TAR insemiconductor fabrication having a refractive index of approximately1.42, which is relatively far removed from the "ideal" of 1.30. As willbe discussed in greater detail hereinbelow, any suitable TAR can beapplied over the substantially conformal layer of photoresist appliedaccording to the present invention, to compensate for any lingeringirregularities in the thickness (hence reflectance) of the photoresistlayer.

SOLUTION OF PHOTORESIST

Photoresist is "normally" supplied and applied in a solution ofapproximately 90% (ninety percent) casting solvent, such as ethyllactate, and approximately 10% (ten percent) solids, such as novolacresin (diazoquinone). Such a solution having 90% solvent is entirelysuitable for normal spin-on and other prior art techniques of applyingphotoresist to a substrate (wafer).

According to the invention, a wafer is immersed in a solution ofphotoresist and solvent. The wafer is immersed "face-up" in thesolution.

In one scheme, the solvent is allowed to evaporate, and is preferablycaused to evaporate at an accelerated rate, such as by applying heat. Inthis manner, the solution will become supersaturated (high solidcontent). Contemporaneously (to a limited extent), and subsequently (toa greater extent), upon further evaporation of solvent the photoresistparticles will deposit as a sediment upon the face of the wafer.

FIG. 2a shows apparatus for effecting this method of depositingphotoresist upon the face of a wafer.

A semiconductor wafer 202 is immersed in a reservoir 204 containing asolution 206 of photoresist solids and solvent. In this example, thewafer is disposed flat, face up, at the bottom of the reservoir.

In order to apply the photoresist solids as a conformal layer over thefront (top, as viewed) surface of the wafer, the solvent is allowed orcaused to evaporate out of the solution 206. Preferably, a heatingapparatus 208 accelerates the evaporation of the solvent from thesolution.

Evidently, a known quantity of photoresist solids are contained in thesolution above (as viewed) the wafer. And this quantity of photoresistsolids is readily established to be sufficient to form a photoresistlayer of desired thickness on the wafer. Preferably, there are more thanenough solids contained in the solution above the wafer to form thedesired layer. Hence, it is empirically determined how long the waferneeds to be in the solution in order to achieve a sedimentary layer ofphotoresist of desired thickness. This is referred to as the "residencetime" of the wafer in the solution.

Preferably, the solution 206 is supersaturated with photoresist solids.For example, a solution having on the order of equal amounts (50%, 50%)of solids and solvent is created. Prior to immersion of the wafer insuch a supersaturated solution, the solution is maintained at anelevated temperature, and is covered (not shown) so that the solventdoes not evaporate out to a significant degree. (The boiling pint ofethyl lactate is 154°.) Then, the wafer is immersed in the solution.Then, the temperature of the solution is allowed (or caused) to drop.This will accelerate the sedimentary deposition of photoresist particles(solids) onto the face of the semiconductor wafer.

Rather than simply allowing/causing the solvent to evaporate out of thesolution, it is also possible to "force" the photoresist solids out ofsolution. Generally, a solvent such as ethyl lactate has a relativelyhigh "polarity". By adding an "anti-solvent" to the solution, thepolarity of the solvent is neutralized ("reverse biased"), and thephotoresist material will sediment out of solution. Hexane, beingrelatively non-polar, will cause this effect. A number of othernon-polar materials may be added to the solution to cause thephotoresist particles to sediment out of solution. Again, the residencetime of the wafer in the solution (after adding the anti-solvent) mustbe limited to control the thickness of the photoresist layer depositedon the wafer.

FIG. 2b shows the resulting conformal layer of photoresist 212sedimentarily deposited over a portion of the topographical wafer 202.(This figure is also illustrative of the end result of otherembodiments.)

By allowing/causing photoresist particles to deposit themselvessedimentarily upon a topological (irregular surface) wafer, asubstantially conformal (uniform thickness) layer of photoresist iscreated. Hence, reflectivity of the layer is uniformized. Nevertheless,there may be minor variations in the photoresist thickness that willmanifest themselves in non-uniform reflectivity, as discussed above.

Therefore, it is also advantageous to apply a suitable topantireflective (TAR) coating, such as IBM's TAR, over the substantiallyconformal layer of sedimentarily-deposited photoresist. An exemplary TARis shown in FIG. 2b as thin film 214.

SUSPENSION OF PHOTORESIST

In this embodiment, photoresist particles are suspended (i.e., notdissolved) in an inert (vis-a-vis the photoresist solids) medium, suchas water, preferably deionized water.

As in the previous embodiment, a wafer is immersed in the suspension ofphotoresist and water. Again, the wafer is immersed "face-up" in thesuspension. The water may simply be allowed to evaporate, and may alsobe caused to evaporate at an accelerated rate, such as by applying heat.In relative terms, however, this may take an intolerable amount of time.Preferably, the photoresist particles are caused to sediment out ofsuspension more quickly.

According to the invention, photoresist particles are "artificially"maintained in suspension (in water) by agitating the suspension. A waferis immersed in the suspension, agitation is ceased, and the suspensionis allowed to become quiescent, at which point the photoresist particleswill sediment out of the suspension onto the face of the wafer.

According to the invention, the photoresist particles are maintained insuspension by agitating the suspension with any suitable means, such aswith an ultrasonic transducer.

FIG. 3 shows apparatus for effecting this method of depositingphotoresist upon the face of a wafer.

A semiconductor wafer 302 is immersed in a reservoir 304 containing asuspension 306 of photoresist solids and water. In this example, thewafer disposed at the bottom of the reservoir. An ultrasonic transducer308 is mounted to the reservoir, and turned on, to keep the photoresistsolids in suspension.

The wafer is immersed in the suspension, again at the bottom of thereservoir, the ultrasonic transducer is turned off, and photoresistsolids are allowed to precipitate (sediment) onto the face of the wafer.Again, a sufficient quantity of photoresist solids are available in thesuspension to create a conformal photoresist layer on the wafer ofdesired thickness, and the residence time of the wafer in the reservoir(after turning off the transducer) is controlled.

Advantageously, when the wafer is withdrawn from the reservoir, a"residual" thin film of water may remain over the sedimentary layer ofphotoresist on the wafer. As mentioned above, water has a refractiveindex well suited to behaving as a top antireflective coating overphotoresist. Hence, by depositing photoresist as a sediment from awater-based suspension of photoresist particles, it is possible toachieve a substantially conformal (uniform thickness) layer (film) ofphotoresist, as well as provide a top anti-reflective coating in thesame process. The resulting structure will resemble that shown in FIG.2b.

In either case, solution or suspension (together referred to as"mixture"), the wafer is withdrawn from the mixture when it isdetermined that a sufficient amount of photoresist solids haveprecipitated onto the face of the wafer. This may be accomplished bywithdrawing the wafer from the mixture (at the appropriate time), or by"decanting" the mixture from the reservoir (again, at the appropriatetime). Either of these techniques are straightforward, and will beunderstood by those skilled in the art to which the present inventionmost nearly pertains.

MECHANISM FOR SUPPORTING WAFER

In the techniques discussed above, the semiconductor wafer is simplyplaced flat on the bottom of the reservoir, and it is ensured that thereare sufficient, typically much more than sufficient, solids present inthe mixture (solution or suspension) above the wafer to deposit aconformal layer of desired thickness. The need to limit the time thatthe wafer is exposed to sedimentary deposition from the mixture may beviewed as a shortcoming. Hence, it is proposed to avoid thisshortcoming.

In any given mixture (solution or suspension) of photoresist solids, itcan be assumed that the distribution of solids is fairly uniformthroughout the mixture (especially if the mixture is agitated). It canalso be determined how much, up to 100%, of the photoresist solids willprecipitate out of the mixture (i.e, when an anti-solvent is added, whenthe ultrasonic transducer is turned off, etc.).

According to this embodiment of the invention, the wafer is supported ata known, elevated position within the reservoir so that there is aportion of the mixture below the wafer and a portion of the mixtureabove the wafer. The latter is of primary concern. The elevation of thewafer is determined to be at a position where there is just enoughmixture above the wafer so that whatever percentage of solids areexpected to precipitate out of the mixture will correspond to a giventhickness of the desired conformal layer on the wafer.

FIG. 4 shows apparatus for accomplishing this objective. A semiconductorwafer 402 is supported, in a reservoir 404 containing a mixture 406 ofphotoresist solids, on a platform (stage, carrier) 408. The wafer stage408 is mechanically connected with a suitable linkage 410 to anactuator, such as a stepper motor 412. The motor 412 is capable ofpositioning the wafer at any predetermined level (height, as viewed)within the reservoir.

Notably, the wafer 402 is positioned within the reservoir at a positionwhere there is a known quantity of mixture 406, containing a knownquantity of dissolved or suspended photoresist solids, in a portion ofthe mixture above (as viewed) the wafer. This known quantity of solidscorresponds to a desired thickness of the conformal photoresist layersought to be deposited on the wafer.

Employing any of the mixtures/techniques described above for forming asedimentary layer of photoresist on the face of the wafer, the wafer isallowed to reside in the mixture for a period of time sufficient toensure full (rather than partial, as was described above) precipitationof the photoresist solids onto the face of the wafer. Evidently, onlythose solids contained in the portion of the mixture above the waferwill deposit themselves onto the face of the wafer. At the end of thistime, the wafer is withdrawn from the mixture, by actuating the motor412.

COMPENSATING FOR THICKNESS VARIATIONS

Notwithstanding the above, assuming that a perfectly conformal,perfectly uniform thickness of photoresist is not obtained, and thereare lingering nonuniformities in reflectance manifesting themselves inunacceptable cd variations, and notwithstanding compensating for thesereflectance nonuniformities by applying an overlying TAR, we are leftwith a non-uniform layer of photoresist exhibiting nonuniformreflectance, and the cd problems that ensue. Albeit, thenon-uniformities may be much less than could otherwise (withoutemploying sedimentary deposition of photoresist) have been obtained.

As mentioned above, based on a general knowledge of the characteristicsof large troughs (or islands), vis-a-vis small troughs, the reflectanceissue can be addressed in a general way by differentially biasing thephotomask--namely, making some lines in the mask bigger (or smaller, asthe case may be) to compensate for nonuniform reflectance and nonuniformabsorption of energy (photolithography illumination). This is, ofcourse, based largely on large area assumptions.

U.S. Pat. No. 4,977,330 discloses an in-line photoresist thicknessmonitor. A determination of photoresist thickness is made by fiberoptics illuminating the coated wafer and measuring light scattered backfrom each illuminated portion of the wafer. This patent is cited as anexample of existing equipment which can measure photoresist thickness.

According to the invention, thickness variations in the photoresistlayer are measured and mapped (i.e., recorded, such as in a computermemory). This information about photoresist thickness is used todetermine (calculate and map) the reflectivity, point-to-point, of thephotoresist layer (see FIG. 1c, above). The reflectivity data is used tocontrol the exposure dose (intensity x time) of subsequentpoint-by-point (versus blanket) optical semiconductor fabricationprocesses, such as e-beam lithography, or the like. At points on thewafer exhibiting greater reflectance, the exposure dose is increased,and vice-versa. In this manner, the resulting absorption by thephotoresist layer can be uniformized, point-to-point, irrespective ofthickness (reflectance) variations in the photoresist layer. Hence,better control over the cd of underlying features being fabricated canbe effected.

In a variation on the theme of measuring thickness and calculatingreflectance, the reflectance can be measured directly and used tocontrol e-beam (or the like) exposure dose.

FIG. 5 shows a semiconductor wafer 502, assumed to have a nonconformallayer of photoresist on its top (as viewed) surface. A beam 504 from alaser apparatus 506 is scanned, point-by-point, across the front surfaceof the wafer. Using a suitable angle of incidence for the beam, a largepercentage of the beam will be absorbed by the photoresist layer. (Ofcourse, the intensity and wavelength of the laser must be selected so asnot to damage the semiconductor devices on the wafer.) However, inproportion to the reflectance of the wafer at any given point, some ofthe beam 504 will be reflected off of the front surface of the wafer, asindicated by the dashed line 508. The intensity of the reflected beam508 is readily measured by a dosimeter 510, or the like.

The laser is under the control of a computer 512, or the like, whichdirects the scanning of the beam in any known manner, such as rasterscanning. Hence, the position of the beam 504 is well controlled.Simultaneously, the output of the dosimeter 510 is monitored by thecomputer 512, which then creates a "map" (not shown) of reflectanceversus position on the wafer. This map is used to control the exposuredose of subsequent optical processing apparatus, such as e-beamlithography, as discussed above.

ALTERNATE EMBODIMENT

Hereinabove, it has been described how a semiconductor wafer, orsubstrate, can be provided with a conformal layer of photoresistdeposited on the substrate, wherein the layer of photoresist has asubstantially uniform thickness over substantially the entire topsurface of the substrate. And, it has been discussed how this desiredresult can be achieved by immersing a wafer in a reservoir. By way ofreview, a substrate (wafer) initially has a generally flap top (front)surface. Features, such as oxide regions, poly gates, and the like, areformed on the surface of the substrate, and create topographicalirregularities on the top surface of the substrate. It should berealized that depositing photoresist over such a substrate, whetherconformally or not, is an interim step in the processing of thesubstrate. In this regard, the wafer (substrate) can be considered to be"in-process" when the photoresist is applied.

According to an embodiment of the invention, the wafer is not immersedin a reservoir containing a mixture (solution or suspension) ofphotoresist. Rather, the mixture is applied, such as with spin-ontechniques, directly to the surface of the wafer.

FIG. 6 shows a technique for providing a conformal layer of photoresistover a wafer, without a reservoir. A wafer 602 is mounted, such as byvacuum chucking, to a rotatable platen 604. A thickened mixture ofphotoresist is applied to the front surface of the wafer from a supply606. The thickened mixture is shown at 608, exiting the supply 606 ontothe front surface of the wafer 602. The platen is rotated, so that themixture spreads itself over the front surface of the wafer.

The thickened mixture is sufficiently thick so as to stay on the frontsurface of the wafer. Surface tension and adhesion will ensure that aquantity of the thickened mixture will reside on the surface of thewafer. For example, a high solid content solution of photoresist can beapplied in this manner. In a manner similar to that described withrespect to the previous embodiments, the photoresist solids areallowed/caused to sediment (precipitate) out of the mixture, in thiscase the mixture on the surface of the wafer, so that the solids aredeposited as a conformal layer on the surface of the wafer. For example,a solution of photoresist is applied from a supply to the surface of thewafer, the supply is removed, and an anti-solvent is applied (fromanother, similar supply, not shown) to the mixture on the surface of thephotoresist to cause the photoresist to sediment. In this manner, theuse of a reservoir is avoided.

What is claimed is:
 1. Method of compensating for nonuniform reflectancein a photoresist layer on a semiconductor wafer, for subsequentpoint-to-point optical processing, comprising:determining thereflectance of the photoresist layer at a plurality of points on thewafer; mapping reflectance versus position for the plurality of points;adjusting the exposure dose of subsequent optical processing, on apoint-by-point basis, based on the reflectance at any given point. 2.Method according to claim 1, wherein:the optical processing is e-beamlithography.
 3. Method according to claim 1, furthercomprising:measuring the thickness of the photoresist layer, on apoint-by-point basis; and calculating the reflectance based on themeasure thickness.
 4. Method according to claim 1, furthercomprising:measuring the reflectance directly.
 5. Method according toclaim 4, further comprising:scanning a laser beam across the wafer; andmeasuring the intensity of the beam as reflected from the surface of thewafer.